How to convert canister powder loads to surplus values

I've got a few rifles with good loads worked up for them, using mostly Hogdon Retumbo. Problem is, I pay between $30-$40 a pound for it in Canada and often have trouble getting it at all. So, I've ordered a keg of WC 860 & WC 868. I've got to pick it up in person, but the price is great.

Will WC860 & WC868 work in place of the Retumbo in these loads??
What can I use as a refernce point to start from with these powders, I understand that it is best to reduce 15% and work up because of more lot to lot variation than canister powders.

There is no conversion. You will need to start your load development all over, which is what you must do anytime you change a component. It should really be done anytime you use powder from a different lot, this is why many reloaders buy large lots of matching powders.

That is what I was afraid of. I'll have to use whatever H870 data for WC868 or AA8700 data for WC860 then reduce by 15% from posted max load and work up watching for pressure signs on the primer, case head/web area & bolt lift? Thanks for the help, I'll enjoy working up the new loads. If I don't get the time to do it before season I'll just use the loads I have on hand or use up the last 3500 grns retumbo I've got left.
I'm going to get my powder end of July, so I've got my work cut out for me. From what is posted on here I'll have to work up a cold weather load as well because hunting season temp ranges between +5C/40F down to -25C/-20F. Oh well, might have to try some of our local Canadian Wildcat bullets too while I'm doing load development. I was thinking of trying the 150grn .264 bullets.